Python

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Python Getting Started

• July 12 - July 14, 2011

• Drew Flater, Ghislain Prince

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Does this describe you?

• New to Python scripting
• Comfortable using ArcGIS but want to become more
  efficient
• Moving to Python from other scripting language
• Interested in whats new in ArcGIS 10

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Agenda

• Python scripting essentials
• What is Python? Why use Python scripting?
• Python 101
• What is ArcPy?
• Scripting geoprocessing tools
• Error handling
• ArcPy functions
• Batch processing
• Python script tools

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Python Scripting Essentials
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What is Python?

• Python is an easy to learn, powerful language
  (with) high-level data structures and a simple
  but effective approach to object-oriented
  programming. Pythons elegant syntax and dynamic
  typingmake it an ideal language for scriptingin
  many areas and on most platforms. python.org
• Scripting language of ArcGIS
• Free, cross-platform, easy to learn, great
  community

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Why use Python scripting?

• There are many ways to run single tools
• Use scripting to develop, execute, and share
  geoprocessing workflows
• Improves productivity

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Python 101

• Where do I write Python code?
• IDE like PythonWin or PyScripter
• Python window in ArcGIS
• How do I run a Python script?
• Double-click or cmd prompt
• What are variables?
• A name that stores a value assigned using

input "C/Data/Roads.shp" distance 50 both
input, distance Variables act as substitutes
for raw values arcpy.Buffer_analysis(input,
"Roads_buffer.shp", distance)
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Python 101

• Python has logic for testing conditions
• if, else statement
• Colon at end of each condition
• Indentation determines what is executed
• tests equality other operators like gt, lt, !

var "a" if var "a" Execute indented
lines print("variable is a") else
print("variable is not a")
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Python 101

• Techniques for iterating or looping
• While loops, for loops
• Colon at end of statement
• Indentation determines what is executed

x 1 while x lt 5 print x x x 1 x 1,
2, 3, 4 for num in x print num
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Python 101

• Case sensitivity
• Variables, functions, etc. are case sensitive
• name X is not defined, function X does not
  exist
• Building blocks
• Function a defined piece of functionality that
  performs a specific task requires arguments ()
• Module a Python file where functions live
  imported
• Package a collection of modules
• math.sqrt(100) 10.0
• Theres a module for that

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ArcPy

• The access point to geoprocessing tools
• A package of functions, classes and modules, all
  related to scripting in ArcGIS
• Helper functions that enable workflows
  (ListFeatureClasses, Describe, SearchCursor, etc)
• Classes that can be used to create complex
  objects (SpatialReference, FieldMap objects)
• Modules that provide extended functionality
  (Mapping, SpatialAnalyst modules)
• Enhancement of arcgisscripting module (pre-10.0)
• Your old scripts will work

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ArcGIS Python window

• Embedded, interactive Python window within ArcGIS
• Access to ArcPy, any Python functionality
• Great for experimenting with Python and learning
  tool syntax
• Help pane

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Scripting geoprocessing tools

• Demo

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Executing a tool in Python

• ArcPy must be imported
• Follow syntax arcpy.toolname_toolboxalias()
• Enter input and output parameters

Import ArcPy import arcpy Set workspace
environment arcpy.env.workspace "C/Data"
Execute Geoprocessing tool arcpy.Buffer_analysis("
Roads.shp", "Roads_buffer.shp", "50 Meters")
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Getting tool syntax

• Tool help page
• Copy as Python Snippet
• Export Model to Python script
• Drag tool into Python window
• help(arcpy.Buffer_analysis)

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Setting environments in Python

• Accessed from arcpy.env
• Workspace, coordinate system, extent, etc.
• Global parameter
• See tool help for honored environments
• Provides finer control of tool execution
• Makes scripting easier

arcpy.env.workspace "C/Data" arcpy.env.extent
"0 0 100 100"
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Error handling

• Why do errors occur?
• Incorrect tool use
• Typos
• Syntax errors
• What can I do if my script doesn't work?
• View the geoprocessing messages
• Use Python error handling
• Debug the script in an IDE

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Geoprocessing messages

• Three types of messages
• Informative messages
• Warning messages
• Error messages
• Displayed in the Python window
• arcpy.GetMessages()
• () All messages
• (0) Only informative messages
• (1) Only warning messages
• (2) Only error messages

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arcpy.GetMessages

• Execute Geoprocessing tool
• arcpy.Buffer_analysis("Roads.shp",
  "Roads_buffer.shp", "50 Meters")
• Print the execution messages
• print(arcpy.GetMessages())
• gtgtgt
• Executing Buffer Roads.shp Roads_buffer.shp '50
  Meters'
• Start Time Tue July 12 085240 2011
• Executing (Buffer) successfully.
• End Time Tue July 12 035245 2011(Elapsed Time
  5.00

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Python error handling

• TryExcept
• "Try to do something, and if an error occurs, do
  something else!"

Start Try block try arcpy.Buffer_analysis("
Roads.shp", "Roads_buffer.shp", 50 Meters)
If an error occurs except Print that
Buffer failed and why print("Buffer failed")
print(arcpy.GetMessages(2))
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Error handling

• Demo

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ArcPy Functions
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ArcPy functions

• Helper functions
• Perform useful scripting tasks
• Print messages (GetMessages)
• List data to perform batch processing
  (ListFeatureClasses, 12 total List functions)
• Getting data properties (Describe)
• Etc.
• Supports automation of manual tasks

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Batch processing

• Run a geoprocessing operation multiple times with
  some automation
• Clip every feature class in a geodatabase to a
  boundary
• Calculate statistics for every raster in a folder
• List functions used in Python
  to perform batch processing
• One of the highest values in
  scripting

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arcpy.ListFeatureClasses

• Set the workspace
• arcpy.env.workspace "C/Data/FileGDB.gdb/FDs"
• Get a list of all feature classes
• fcList arcpy.ListFeatureClasses()
• Print the list of feature classes one at a time
• for fc in fcList
• print(fc)

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ArcPy functions -- Describe

• Use the Describe function to read data properties
• Returns an object with properties

• Allows script to determine properties of data
• Data type (shapefile, coverage, network dataset,
  etc.)
• Shape type (point, polygon, line, etc.)
• Spatial reference
• Etc.

• Describe a feature class
• desc arcpy.Describe("C/Data/Roads.shp")
• print(desc.shapeType)
• gtgtgt "Polyline"

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Batch Processing

• Demo

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Python script tools

• Connects Python to ArcGIS
• Best way to create and share custom workflows
• More accessible than stand-alone Python script
• Extends ArcGIS
• Integrated with geoprocessing framework
• Inherits geoprocessing properties and
  environments from application
• Can be used in ModelBuilder
• Works with map layers

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Receiving arguments

• Arguments are user-defined inputs to a script
• Values passed to script from user, instead of
  hard-coded
• Use GetParameter or GetParameterAsText to read
  arguments

• Create variables from input arguments
• inputFC arcpy.GetParameterAsText(0)
• outputFC arcpy.GetParameterAsText(1)
• First and third parameters come from arguments
• arcpy.Clip_analysis(inputFC, "C/Data/boundary.shp
  ", outputFC)

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Connecting parameters
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Script tools

• Demo

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Python scripting resources

• ArcGIS Resource Center
• resources.arcgis.com
• Online documentation
• Geoprocessing script gallery, blog, tutorials,
  presentations
• python.org
• Python References
• Learning Python by Lutz, et al.
• The Python Standard Library by Example by
  Hellmann
• diveintopython.org

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Esri Training for Python

• esri.com/training

• Instructor-Led Course
• Introduction to Geoprocessing Scripts Using
  Python
• Web Course (free)
• Using Python in ArcGIS Desktop 10

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Other Python sessions

• Python Intermediate
• - Following this session
• Python Raster Analysis
• - Wed 315pm Room 5 A/B
• Python - Scripting for Map Automation
• - Wed 315pm Room 9
• Building Tools with Python
• - Thu 1015am Room 9
• Spatial Analysis Island demo theater

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Thanks for your feedback!www.esri.com/sessioneval
s
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